The present invention relates to a brake booster system for providing intensified braking forces by boosting brake operating force such as pedal pressure, more particularly to a brake booster system capable of providing intensified braking forces with allowing reduced input stroke of a brake operating member such as a brake pedal.
Conventionally, most vehicles such as automobiles employ a brake booster system, which enables intensified braking forces to be produced by small pedal pressure on a brake pedal, in the brake system thereof. There is a hydraulic booster system, as an example of the conventional brake booster systems, which intensifies pedal pressure by fluid pressure of hydraulic fluid to output intensified forces as shown in FIG. 17. In this figure, numeral 1 designates the hydraulic brake booster system, numeral 2 designates a brake pedal, numeral 3 designates a hydraulic booster, numeral 4 designates a pump, numeral 5 designates a motor M, numeral 6 designates an accumulator, numeral 7 designates a booster reservoir, numeral 8 designates a master cylinder (hereinafter, sometimes referred to as "MCY"), numeral 9 designates a MCY reservoir, and numeral 10 designates each wheel cylinder (hereinafter, sometimes referred to as "W/C").
In such a hydraulic brake booster system 1, the pump 4 is actuated by driving the motor 5 whereby the hydraulic fluid in the booster reservoir 7 is transferred to the hydraulic booster 3 and the accumulator 6 in such a manner that hydraulic fluid in predetermined pressure is conserved in the accumulator 6. In this state, by pedaling the brake pedal 2, a control valve (not shown) of the hydraulic booster 3 is switched so that the pressurized hydraulic fluid in the accumulator 6 is introduced into a power house (not shown) of the hydraulic booster 3 corresponding to the pedal pressure. By the hydraulic fluid introduced into the power house, a power piston (not shown) boosts the pedal pressure to output the intensified force. Then, the piston of the MCY 8 is actuated by the output of the hydraulic booster 3 so that MCY 8 produces MCY pressure Pi which is supplied to the W/Cs 10 as brake fluid pressures to put on the brakes. Since the pedal pressure is intensified by the hydraulic booster 3 at this point, the braking force produced is greater.
There is also a vacuum brake booster system, as another example of the conventional brake booster systems, which intensifies pedal pressure by vacuum to output intensified forces as shown in FIG. 18. In this figure, numeral 11 designates a vacuum brake booster system and numeral 12 designates a vacuum booster.
In such a vacuum brake booster system 11, the vacuum booster 12 is provided with a diaphragm piston (not shown) separating a constant pressure chamber (not shown) in which vacuum is always introduced, from a variable pressure chamber (not shown). By pedaling the brake pedal 2, a switch (not shown) of the vacuum booster 12 is switched so that outside air is introduced into the variable pressure chamber according to the pedal pressure. By the outside air introduced into the variable pressure chamber, the diaphragm piston boosts the pedal pressure to output intensified forces. Then, the piston of the MCY 8 is actuated by the output of the vacuum booster 12 in the same manner so that MCY 8 produces MCY pressure Pi which is supplied to the W/Cs 10 to put on the brakes. Since the pedal pressure is intensified by the vacuum booster 12 at this point, the braking force produced is greater.
By the way, in the hydraulic brake booster system 1 and the vacuum brake booster system 11 as mentioned above, the hydraulic booster 3 and the vacuum booster 12 are relatively large-sized and costly.
One of conceivable systems capable of solving this problem is a brake booster system which intensifies MCY pressure produced by pedal pressure to output intensified braking forces without using such a hydraulic booster 3 or vacuum booster 12, as shown in FIG. 19.
As shown in FIG. 19(a), the brake booster system is provided with a pressure-intensifying valve 13 on a brake pressure line connecting the MCY 8 and the W/C 10. The pressure-intensifying valve 13 comprises a valve piston 14 having a valve portion 14a and a rubber valve seat 15 in which the valve portion 14a can be seated. In this case, the effective receiving area of the valve piston 14 at the MCY 8 side when the valve portion 14a is seated in the valve seat 15 is set to S1 and the non-receiving area of the valve piston 14 is set to S2 in such a manner that the area S1 is greater than the area S2. As the pressure-intensifying valve 13, a proportioning valve (hereinafter, sometimes referred to as "P valve") conventionally used for pressure reducing control of the brake pressure on the rear wheels may be used, with its input/output being reversed, for properly adjusting the distribution of braking forces on the front and rear wheels at MCY pressure exceeding a predetermined values. However, the pressure-intensifying valve 13 is not limited thereto.
The system is also provided with a pressure-intensifying pump 17, which is driven by a motor 16 to supply brake fluid of the MCY 8 to the W/C 10, and a pedal switch 18 for detecting pedaling on the brake pedal 2. The pressure-intensifying pump 17 is disposed on a line in parallel with the line on which the pressure-intensifying valve 13 is disposed in such a manner as to by-pass the pressure-intensifying valve 13.
In the brake booster system, as the pedal switch 18 detects that the brake pedal 2 is pedaled, the motor 16 is actuated to drive the pressure-intensifying pump 17. In addition, as MCY pressure is developed in the MCY 8 by pedaling the brake pedal 2, the valve piston 14 immediately slides so that the valve portion 14a is seated in the rubber valve seat 15. The relation between the MCY pressure Pi at the input of the pressure-intensifying valve 13 and the WIC pressure Po at the output of the pressure-intensifying valve 13 (while Po designates discharge pressure of the pressure-intensifying pump 17 in fact, it also designates W/C pressure when the W/C pressure is equal to the discharge pressure, and as will be described later, the W/C pressure is designated by Pw when the W/C pressure is different from the discharge pressure.) can be given by:
Expression 1! EQU Po={S1/(S1-S2)}Pi
wherein
S1=the receiving area of the valve plunger 14 at the MCY 8 side when the valve portion 14a is seated in the valve seat 15; and PA1 S2=the cross-sectional area of a non-receiving portion 14b of the valve plunger 14.
When the valve portion 14a is seated in the rubber valve seat 15, the W/C pressure Po is intensified by the pressure-intensifying pump 17. When the W/C pressure Po exceeds the pressure Po given by the expression 1, the valve piston 14 is returned so that the valve portion 14a moves away form the rubber valve seat 15. Therefore, the W/C pressure Po escapes to MCY and thus falls. When the W/C pressure Po reaches the pressure Po given by the expression 1 again, the valve piston 14 slides rightward again so that the valve portion 14a is seated in the rubber valve seat 15, thereby offering a balance. In this manner, the W/C pressure Po is intensified to rise relative to the MCY pressure Pi as shown by a solid line in FIG. 19(b) in such a manner as to satisfy the expression 1.
As mentioned above, according to the brake booster system shown in FIG. 19, the MCY pressure produced by the pedal pressure can be intensified without using any booster.
However, in such a brake booster system, when the W/C pressure in the W/Cs 10 is intensified by the pressure-intensifying valve 13 and the pressure-intensifying pump 17, incremental pressure should be supplemented by the brake fluid discharged from the MCY 8, with the result that the stroke of the brake pedal 10 must be increased. That is, since the pedal stroke is defined according to the fluid stiffness of the W/Cs 10 and the like, the pedal stroke can never be shortened.
In also the conventional hydraulic brake booster system 1 and the vacuum brake booster system 11 mentioned above, since the pedal stroke is defined according to the fluid stiffness of the W/Cs 10 and the like in the same manner, there is a problem that the pedal stroke can not shortened.